To make the best use of the limited funding for biodiversity conservation, resources should be used cost-effectively. Support to organic farming is a widely implemented strategy to enhance farmland biodiversity, yet its cost-effectiveness for biodiversity conservation remains largely unexplored. Using an ecological-economic model calibrated with data from southern Sweden, we evaluated the cost-effectiveness of organic farming for enhancing biodiversity in landscapes of varying agricultural productivity (low and high). We focused on flowering plant diversity, which further has an essential role in supporting pollinators and natural enemies of crop pests. Our findings reveal that organic farming generally proves more cost-effective for enhancing biodiversity in high-productive compared to low-productive agricultural landscapes. We also found that depending on landscape characteristics, conversion of 10–20 % of arable land to organic management could substantially increase species richness at a relatively low cost (<6 % reduction in farmers' profits). However, with further increases in conversion rates, costs escalated exponentially while the benefits to biodiversity diminished. Across larger spatial scales (1000 to 2000 ha), biodiversity peaked when land was approximately evenly divided between conventionally and organically managed farms, owing to species turnover dynamics. Our study underscores the importance of tailoring policies to incentivize organic farming where it has the greatest impact on biodiversity. Importantly, it suggests that policies aimed at incentivizing organic farming in high-productive regions could be more cost-effective for biodiversity conservation than current policies that favour conversion in low-productive regions.